Both hard and so-called soft contact lenses have been described in the prior art. For example, U.S. Pat. No. 3,503,393 to Siederman, as well as U.S. Pat. No. 2,976,576 to Wichterle, describe processes for producing hydrophilic polymers of poly and (hydroxy-ethyl methacrylate) in aqueous reaction media having a sparingly cross-linked polymeric hydro-gel structure and having the appearance of elastic, soft, transparent hydrogels. Other soft contact lenses include those produced from silicones and other optically suitable materials.
The primary virtues of these lenses are their softness and optical suitability. These hydrophilic lenses are particularly desirable in opthalmology due to their quite remarkable ability to absorb water with a concomitant swelling to a soft mass of extremely good mechanical strength, complete transparency and an ability to retain shape and dimensions when subjected to equilibration in a given fluid.
One of the problems associated with such soft contact lenses, as well as hard contact lenses, is the method of their sterilization and cleaning. The very property of the hydrophilic soft lenses which allows them to absorb up to about 150 percent by weight of water also allows preservatives, which might otherwise be used for cleaning and sterilization to be absorbed, even concentrated and later released when the lens is positioned in the eye of the user. The release may be much slower than the uptake, thereby causing a buildup of the preservative material in the lenses. Such buildup eventually adversely affects the physical characteristics of the lenses including dimension, color, and the like. This can exhibit the harmful result of damaging or staining the contact lens itself and/or harming the delicate tissues of the conjunctivae and/or cornea of the user.
Hard contact lenses do not absorb appreciable amounts of water, only on the order of from about 0.1 to 0.4 percent by weight; consequently, the employment of effective preservatives does not create problems of the magnitude attendant soft contact lenses. Users of soft contact lenses are cautioned that solutions designed for use with hard contact lenses should be avoided, for the reason that the preservatives present in such solutions will be absorbed, even concentrated by the soft lens material, and may seriously damage the lens and/or tissue of the eyes of the user.
U.S. Pat. No. 3,689,693 discloses a process of soaking and sterilizing hydrophilic soft contact lenses. Therein, it is disclosed that a number of anti-microbial agents may be absorbed and concentrated in soft lenses, also suggesting that these materials may cause corneal damage and that similar in vitro and in vivo testings have demonstrated the undesirability of such anti-microbial agents when used with hydrophilic lenses.